An average human, utterly unremarkable in every way, can
perceive a million different colors. Vermilion, puce, cerulean, periwinkle, chartreuse—we have thousands of words for them, but mere language can never capture our extraordinary range of hues. Our powers of color vision derive from cells in our eyes called cones, three types in all, each triggered by different wavelengths of light. Every moment our eyes are open, those three flavors of cone fire off messages to the brain. The brain then combines the signals to produce the sensation we call color.
Vision is complex, but the calculus of color is strangely simple: Each cone confers the ability to distinguish around a hundred shades, so the total number of combinations is at least 1003, or a million. Take one cone away—go from being what scientists call a trichromat to a dichromat—and the number of possible combinations drops a factor of 100, to 10,000. Almost all other mammals, including dogs and New World monkeys, are dichromats. The richness of the world we see is rivaled only by that of birds and some insects, which also perceive the ultraviolet part of the spectrum.
Researchers suspect, though, that some people see even more. Living among us are people with four cones, who might experience a range of colors invisible to the rest. It’s possible these so-called tetrachromats see a hundred million colors, with each familiar hue fracturing into a hundred more subtle shades for which there are no names, no paint swatches. And because perceiving color is a personal experience, they would have no way of knowing they see far beyond what we consider the limits of human vision.
Over the course of two decades, Newcastle University neuroscientist Gabriele Jordan and her colleagues have been searching for people endowed with this super-vision. Two years ago, Jordan finally found one. A doctor living in northern England, referred to only as cDa29 in the literature, is the first tetrachromat known to science. She is almost surely not the last.
The first hint that tetrachromats might exist came in a 1948 paper on color blindness. Dutch scientist HL de Vries was studying the eyes of color-blind men, who, along with two normal cones, possess a mutant cone that is less sensitive to either green or red, making it difficult for them to distinguish the two colors. He tested their vision by having them perform a basic matching task. Twiddling the dials on a lab instrument back and forth, the men had to mix red and green light so that the result, to their eyes, matched a standard shade of yellow. To compensate for their difficulty in discerning hues, color-blind men need to add more green or red than normal trichromats to make a match.
Out of curiosity, De Vries tested the daughters of one subject and observed that even though they were not color-blind—they seemed to distinguish red and green as well as anyone—they needed more red in their test light than normal people to make the match precise. If the women weren’t color-blind, what was going on?
Pondering the situation, De Vries thought he saw an explanation. Color blindness ran in families, affecting men but not women. While color-blind men had two normal cones and one mutant cone, De Vries knew that the mothers and daughters of color-blind men had the mutant cone and three normal cones—a total of four separate cones in their eyes. He suspected the extra cone could be why the women perceived color differently—not because they saw less than most people but because they saw more. He speculated that such women might be using the fourth cone to distinguish more colors than a trichromat, but he buried this insight on the last page of the paper. De Vries never wrote about four-coned women again.